Projection exposure apparatus such as steppers are known in the art. Such apparatus are operable to perform photolithography processes in which a pattern defined on a reticle (i.e., a mask) is transferred onto a semiconductor wafer, glass plate, or other photosensitive substrate. Light used to make the exposure is projected onto the substrate using an optical projection system. Usually, manufacture of semiconductor devices, displays, and the like requires a large number of fabrication steps including separate exposures of different patterns at the same locations on the substrate. Thus, each subsequent exposure made at a location on the substrate requires very accurate alignment and superposition of the present pattern with patterns that were formed on the substrate during previous process steps. Virtually any shifting or distortion of any portion of the actual projected image relative to the intended image cannot be tolerated. This requires extraordinary control of any variable distortion of the projected image.
During the manufacture of a projection-exposure apparatus, it is sometimes necessary to accurately measure the distortion of a projection image formed by the optical projection system and, based on the measurement results, to make an adjustment in the projected-image distortion. Unfortunately, this does not redress the effect of variable (i.e., changes in) distortion arising after such an adjustment is made.
One conventional way in which to adjust the distortion of a projection image is to projection-expose an image of a test pattern, as defined by a "test reticle," onto a test wafer. At various locations in the test image on the test wafer, points of convergence are measured against specifications.
According to another known method of measuring projected-image distortion, locations of various points of convergence (i.e., image points) in an image of the pattern defined by a test reticle are measured directly using a photoelectric sensor movable from one image point to another. Such a method eliminates the need to expose and develop a test wafer.
After measurement of distortion of a projection image on the image surface (i.e., locus of image points at the best-focus position of the projection optical system) of an optical projection system, one conventional way in which to correct an unwanted distortion is to move the projection-optical system relative to the reticle. Alternatively, such correction can be performed by moving a portion of the projection-optical system relative to the image surface. Both methods are used during the manufacture of a projection exposure apparatus.
Projection-image distortion can change due to environmental changes as manifested by dimensional fluctuations of the projection-exposure apparatus. Experience has also revealed that variations in projection-image distortion can be caused by corresponding changes in components such as the reticle. Two methods are known for correcting such problems. In one method, projection-image distortion is either measured or determined as required by calculations performed under various exposure conditions, and then the distortion is corrected for such conditions according to the results of such measurements or calculations. In the other method the projection-image distortion is measured, at fixed intervals of time or whenever exposure conditions are known to have changed, by a photoelectric sensor and then corrected based on the measurement results.
Normally, a projection-optical system is designed so that at least the wafer side of the system is telecentric. In such a system, the illumination light on the wafer side of the projection-optical system is parallel to the optical axis of the system. Thus, the projection magnification does not change upon any displacement of the wafer in the optical-axis direction.
Whenever the telecentricity of the projection-optical system is not constant, telecentricity should be measured along with distortion, and the telecentricity adjusted as required to within specifications.
An example of a method for measuring telecentricity involves measuring the magnification of the projection-optical system at various locations on the optical axis. Telecentricity is calculated from the amount by which magnification changes with a change in measurement location. Based on the calculation results, an optical element or group of elements of the illumination system, for example, can be adjusted to change the telecentricity.
Adjustments for projection-image distortion and for telecentricity can be conducted independently. Usually, the position of the image surface of the projection-optical system is determined first. Then, any distortion is measured at various points on a projected image where the focal point is at the best-focus position. Adjustments are normally made so that the projection-image distortion at the best-focus position is at the lowest achievable level. Likewise, with respect to telecentricity (and without taking into account any projection-image distortion), adjustments are made so that any variance from the ideal constancy of magnification in the optical axis direction is within specifications.
If a variation arising in the imaging position (at a location on a plane substantially perpendicular to the optical axis) of the projected image relative to the best-focus position is the cause of a slope of the main optical axis, then the imaging position of the projection image relative to the shifting from the best-focus position varies linearly. Hence, if the projection-image distortion measured at the best-focus position is adjusted, then the projection-image distortion will effectively be adjusted optimally even if the exposure is made at a location that is displaced from the best-focus position due to an incorrect exposure operation or other process step.
In view of the ever decreasing size of pattern widths which are the exposure target in recent years, the imaging position of the pattern is not only influenced by a slope in the main light beam but also aberrations (frame aberrations and spherical aberrations) of the optical projection system and does not vary linearly with respect to position shifts from the best focus position.
For example, when the wafer shifts from the best focus position in the optical axis direction while the pattern is being exposed onto the wafer, the imaging position will also shift in the same direction regardless of whether the wafer shifts in either the upper or lower direction of the optical axis. Therefore, when the shifting of the focal point position during an actual process exposure is considered as above, a conventional correction method for projection image distortion cannot be said to be a suitable adjustment method for projection image distortion. In other words, there is a defect where the exposure is carried out at a position away from the best focus position, for instance, a position close to the edge of the gradient photo-resist layer on the wafer and not the symmetry centered on the best focus position consequently resulting in the generation of projection image distortion.
Further changes in the illumination conditions to the reticle are made due to the above-mentioned super high resolution technology seen in recent years. At the time these changes are made, the light path of the light beam within the optical projection system varies greatly depending on those illumination conditions. Because of this, optimizing aberrations of the optical projection system and the telecentricity under all conditions is difficult. In addition, the amount of variation of the imaging position when the wafer shifts in the optical axis direction and the shifting from the linearity of that amount of variation increases depending on the illumination conditions resulting in the image distortion of the projection image expanding even more.
Moreover, as stated above, in recent years measurements of projection image distortion have been carried out by using a photoelectric sensor to detect a spatial image of a pattern on a reticle.
When actually exposing a pattern of a reticle onto a test wafer and directly measuring the distortion of the pattern image formed on the test wafer, because there is a thickness to the resist on the wafer, the projection image distortion averaged to a certain degree in the optical axis direction is measured. However, there is a defect wherein when the measurement is carried out by a photoelectric sensor, the measurement is made on one certain plane thereby making it impossible to accurately discover variations of only the projection image distortion in the optical axis direction.
In addition, one approach in recent years for increasing the depth of focus involves a method in which an exposure is made while shifting the substrate to be exposed (wafer) in the optical axis direction. In this system, there is a defect wherein the distortion of the projection image is not sufficiently corrected by only adjusting for the distortion of the projection image at the best focus position because the exposure is carried out in a defocus state.